"We dominated the match and had several chances, the only thing missing was a goal," Brazil's Eder Militao said.
"We should have scored but a lack of attention in one play allowed them to score. It's frustrating considering the way we played."
The match played out in a relatively subdued atmosphere in the cavernous Lusail Stadium, the tournament's biggest venue, with Brazil's reserves eager to impress but Tite's side unable to assert their usual flair.
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Gabriel Martinelli was Brazil's liveliest player and nearly put the five-time champions ahead in the 14th minute when he was picked out by Fred's ball over the top, but the Arsenal forward's header drew a fine reaction save from Devis Epassy.
Cameroon offered nearly no attacking threat until the first half injury time, when Nicolas Moumi Ngamaleu's cross was met with a powerful downward header by Bryan Mbeumo that Ederson clawed away one-handed at full stretch.
Tite had seen enough and made a triple change soon after the interval, bringing on Marquinho for the injured Alex Telles, and replacing Rodrygo and Fred with Everton Ribeiro and Bruno Guimaraes.
The substitutes stirred Brazil into a sudden flurry of chances with Epassy forced into action three times in as many minutes.
First, Martinelli stung the Cameroon keeper's palms with a snap shot in the 56th minute.
Next, Eder Militao tried his luck from the ensuing corner coaxing a fumble out of the Cameroon keeper, who recovered to scramble the ball out to safety, before the lively Antony got in on the act, forcing Epassy into a diving save to keep out his curling effort.
Tite sent on Pedro in the 64th minute and Rafinha in the 79th as Brazil sought to save their perfect record in the group, but Cameroon clung on until Aboubakar arrived in the right place at the right time to seize his moment.